While studying magnetism in physics class this week we learned that when a connecting wire has current flowing through it it has a magnetic force field. We also learned that when two magnetic fields cross each others paths there will be a force which creates motion. I was wondering how a house could be wired without having attraction or repulsion between the electrical wiring.

Question Date: 2006-05-03

Answer 1:

This is a very good question. Indeed, there are forces between wires in a house, sometimes they can be substantial. As it happens, standard wiring occurs in triples: a "hot" wire, a "neutral" wire and a "ground" wire. The anticipated current path is from hot to neutral (neutral indicates that it usually has a potential close to ground). Since the wires are run together, the magnetic field created by one is largely cancelled (opposed) by the other wire. For this reason, the residual field outside of the wire bundle is much smaller than you might expect. The forces are largely between the two wires -- which are usually embedded in polypropylene or Teflon which keeps them from moving. (The ground wire is a redundant safety -- preventing many shocks due to worn cords or appliances). On the other hand, large, high-current wires are run separately, and you have to be careful to anchor them or they will fly around. If you get a chance to see an electric welder, you will sometimes notice the cables will 'jump' at the start and end of welds -- and this is due to those fields.

My great-uncle was and electrical engineer in Colorado when the east and west cost power grids were first connected together. In the initial attempt, the two systems drifted out of sync by 6 degrees -- creating a substantial current flow through the connection. Even though the 'wires' were copper bus bars 9" thick -- they ripped out of their brackets and flew towards each other -- eventually leaving a 2 inch thick puddle of melted copper on the floor. Large electro-magnets have the opposite problem -- their fields press against the windings-- in high field experiments, there are many interlocks to keep the magnet windings from exploding.

You can see the same effects in much smaller scale -- get some copper magnet wire from radio-shack and wind some 2" diameter loops with small numbers (30-100) of turns and then twist the sires together for a few inches away from the coil. You need a power source with constant current capability to make force measurements-- for up to 3A you can do this with an LM-317 and a resistor (also from RS or online). On the other hand, a few D cells in a battery pack can provide intermittent currents of 4-5A. Be sure to not leave them connected for too long -- the wire can get very hot and the batteries will die. In a few minutes, you can verify Faraday's current laws and Lens's repulsion law. Now, if you take a long wire, fold it in half and then wind the pair into a similar coil you will find that it produces almost no residual field when you run a current through it. This is the idea behind paired wire distribution in a house or factory.

Answer 2:

Completely preventing attraction and repulsion as a result of electromagnetic fields is impossible in a system that has electric currents. The way that electricians usually solve this problem is by insulating the wires so that they don't come into physical contact and cause short circuits. Encasing a wire in a box of a conducting material, such as a metal, will also shield the wires inside from magnetic forces outside - but the changing magnetic fields created by the wires turning on and off will cause electric currents to flow around the exterior of the box.